How laser discs work

How they work

Like a CD, a laserdisc has pits and lands back-coated with aluminium foil, but Laserdisc video was stored in analog format. So whereas on an audio CD (or DVD) the pits and lands will signify binary codes, on an LD the pits are created using frequency modulation of an analog signal, with the frequency carrier encoded using pulse-width modulation. The discs were recorded in one of two formats: CAV (constant angular velocity) or CLV (constant linear velocity). CAV discs were spun at a constant rotational speed during playback, with one video frame read per revolution, whereas CLV discs spun progressively slower as the disc was played from inside edge to outside edge. A CAV disc holds up to 30 minutes of content per side, while a CLV disc can hold twice that.

The main advantage of the CAV format was that its simpler playback method allowed "trick play" features such as freeze frame, slow motion, and reverse on all LD players, while CLV discs required a digital frame buffer to perform the same tasks, which was found only in high-end models. Another advantage was to reduce the visibility of cross talk from adjacent tracks, since on CAV discs any crosstalk at a specific point in a frame is simply from the same point in the next or previous frame. The vast majority of titles were only available in CLV.

Laserdisc (left) compared to a DVD.Audio could be stored in either analog or digital format and in a variety of surround sound formats; NTSC discs could carry two analog audio tracks, plus two uncompressed PCM digital audio tracks, which were CD quality. PAL discs could carry one pair, either analog or digital; in the UK the term LaserVision is used to refer to discs with analogue sound, while LaserDisc is used for those with digital audio. Dolby Digital (also called AC-3) and DTS, which are now common on DVD titles, first became available on Laserdisc, and Star Wars: Episode I (1999) which was released on Laserdisc in Japan, is among the first home video releases ever to include 6.1 channel Dolby Digital EX Surround. Unlike DVDs, which carry Dolby Digital audio in digital form, Laserdiscs store Dolby Digital in a frequency modulated form within a track normally used for analog audio. Extracting Dolby Digital from a Laserdisc required a player equipped with a special "AC-3 RF" output and an external demodulator in addition to an AC-3 decoder. The demodulator was necessary to convert the 2.88 MHz modulated AC-3 information on the disc and convert it into a 384 kbit/s signal that the decoder could handle. DTS audio took the place of the PCM audio tracks, and required only a direct connection via Optical Audio cable and a decoder to be heard.

At least where the digital soundtracks were concerned, the level of sound quality was unsurpassed at the time, but the quality of the analog soundtracks varied greatly depending on the disc, and sometimes on the player. Many early and lower-end LD players had poor analog audio sections, and many early discs had poor analog audio tracks, making Digital soundtracks in any form most desirable to serious enthusiasts once they became available. Early Discovision and Laserdisc titles lacked the digital audio option, but many of those movies received digital sound in later re-issues by Universal, and the quality of analog audio tracks generally got better as time went on. Many discs that had originally carried old analog stereo tracks received new Dolby Stereo and Dolby Surround tracks instead, often in addition to a digital track, helping boost sound quality. Later discs also applied CX noise reduction (a type of dynamic compression), which improved the signal-noise ratio of analog audio. In addition many later discs have no analog audio track at all, instead offering the choice of the PCM digital audio track or Dolby Digital.